Return a reference to the value of the previous evaluation -- that is,
a reference to whatever irepl printed after the last Perl you evaluated.
This function will probably evolve to take an argument $n,
to return the $n'th previous result.

With no arguments,
bump $REPL::DEBUG.
With an argument,
set $REPL::DEBUG to that.
This is for debugging iperl itself; currently at 1 it shows eval'd code,
and at 2 it dumps the PPI document corresponding to entered code.